Why table may be outer joined to at most one other table ?

Here is a question which has been boggling me for few days now, and I searched and searched but couldn't find any convincing answer !

Simple question, why is it restricted to have 2 Outer Joins in SQL, on same table even with different columns being used, check the queries below for better understanding. Also I can overcome them using nested sub query or ANSI joins, but then why it is even restricted in the first place using (+) operator!

In this question I'm referring to the error : "ORA-01417: a table may be outer joined to at most one other table"

What I want to ask is :

Why this is allowed :

select * from a, b, c where a.a1 = b.b1 and a.a2 = c.c1

And why this is not allowed:

select * from a, b, c where a.a1(+) = b.b1 and a.a2(+) = c.c1

Please leave ANSI and Nested SubQueries alone :)